1. Field of the Invention
The present invention relates to microwave plasma-enhanced CVD deposition techniques for production of CVD materials such as diamond. More particularly, the present invention relates to methods for controlling and enhancing plasma size and substrate temperature uniformity in microwave plasma-enhanced CVD processes.
2. The Prior Art
Diamond and other films are now routinely deposited using a variety of plasma-enhanced chemical vapor deposition techniques. Of the many techniques available, microwave CVD formation of diamond is particularly advantageous because of the quality of material it produces and in the deposition rates which can be achieved.
There are, however, two problems which limit the utility of microwave-enhanced diamond CVD. These are difficulty in coverage of large areas due to small plasma size, and presence of significant thermal gradients which degrade deposition uniformity. Both of these problems adversely affect diamond growth economies and limit the range of applications for which the material can be used. These problems are also extant in plasma-driven CVD processes for forming other materials.
Certain chemical vapor deposition (CVD) processes proceed by use of a plasma which activates gases and thereby drives the chemistry which results in deposition of the desired material. The excited species created are usually unstable and decay through various processes within a few milliseconds. While plasmas can be ignited and sustained by application of electrical energy through a variety of modalities, a particularly advantageous means is use of microwave radiation, often at a frequency of about 2.45 GHz.
In some CVD processes, notably CVD formation of diamond, deposition chemistry considerations dictate that the process be carried out at gas pressures above 1 Torr. Gaseous mixtures used for diamond growth commonly employ large fractions of hydrogen. Small plasma volumes, typically not larger in diameter than one quarter of the wavelength of the excitation radiation, are produced under these gas mixture and pressure conditions. At 2.45 GHz, therefore, plasmas are typically about 1 inch in diameter. This occurs due to the so-called cavity mode operation, in which standing wave patterns are formed within a resonant cavity, and plasma ignition occurs at high field intensity locations in the standing wave pattern.
The deposition substrate and reaction chamber parts are strongly heated by the plasma fireball, and in many current CVD diamond reactors, this radiation provides all the needed process heating. This heat distribution is strongly nonuniform.
Small plasma size creates a serious problem in that the deposition process varies strongly with respect to proximity to the plasma in a number of critical parameters, including growth rate, diamond quality, and surface structure. These nonuniformities critically affect the technical and economic feasibility of manufacture of various products using diamond films deposited by microwave plasma CVD because they limit the batch size of any CVD run.
Alternative plasma excitation methods are either inferior or do not work. DC glow-discharge allows coverage of large t areas, but at very low growth rates and with material quality lower than that available by microwave means. RF capacitive (parallel-plate) plasma excitation, a common and very useful modality extensively employed in CVD of silicon and other electronic materials, has to date proven incapable of diamond deposition. RF inductive plasmas, while capable of diamond deposition, suffer from non-uniformities due to small plasma size and disadvantageously low growth rates. The ECR (electron cyclotron resonance) mode of microwave plasma excitation can provide extremely large, uniform plasmas, but diamond deposition chemistry is not compatible with the very low pressures at which ECR must be performed (typically 10.sup.-4 Torr or less). Magnetic enhancement of microwave plasmas at pressures above about 10 Torr is not effective.
One means of obtaining larger-sized plasmas is to employ a longer wavelength of excitation radiation. For example, if 915 MHz radiation is used, a larger plasma can be formed because the wavelength is larger than that of the 2.45 GHz radiation described above. This change in wavelength mitigates, but does not abolish, the nonuniformities related to plasma size noted earlier. However, selection of longer wavelength (lower frequency) radiation increases the risk of failure of diamond deposition chemistry due to frequency-related effects. Cost issues arise as well, because lower-frequency sources are generally more expensive than the industrial standard 2.45 GHz sources, and component sizes are larger and more costly.
Another parameter which affects diamond film growth rate and quality is the temperature at which the growth process is performed. This variation leads to thickness and quality nonuniformities which are generally of significant detriment to the economic production of useful diamond films.
Thermal gradients arise in microwave-enhanced diamond CVD systems because substrates are strongly heated by radiation from plasma regions. Because the plasma regions are small and located quite close to the substrate (and may in fact be in contact with the substrate), the rate of local heat delivery to the substrates is greatest nearest the plasma. The substrate region nearest the center of the plasma region is therefore the hottest region.
Thermal radiation effects modify the heat distribution on a substrate undergoing diamond deposition. It is often desirable for engineering reasons to have within the reactor a number of structures which are heated more or less strongly as the deposition proceeds. As the substrate radiates heat, these heated structures re-radiate heat back to the substrate. It is most commonly observed that substrates lose heat more rapidly at their edges (which are often close to cool reactor walls) than at their centers. This encourages development of a center-to edge thermal gradient even in absence of strong gradients imposed by the nonuniform nature of the heat flux from the plasma.
It would therefore be advantageous to provide a means of spreading small cavity-mode plasmas to larger sizes without having to compromise the diamond deposition chemistry and thus the quality of the resulting product.
It would also be advantageous to provide a way to provide a more controlled heating of the substrate material during CVD diamond deposition processes.